AIR RESISTANCE IS THE OPPOSITE REACTION TO A PARACHUTIST'S WEIGHT
WRONG!!!
This is a very common type of misconception
Most students know one of Newton's famous laws (his third) "for every action (force) there is an equal and opposite reaction" but there is often confusion over the reaction!

This law only applies when we think of TWO objects with forces between them:
the reaction to the downwards push of your feet on the floor is the upwards push of the floor on them. (and interestingly enough, this is even true if the floor is giving way or breaking under your weight)
the reaction to your hand pushing a cart is the cart pushing back on you (at the point of contact)
so wherever there are TWO objects interacting (pushing, pulling) then there is an equal and opposite reaction...note that the reaction acts on a different object to the action.

When an object is travelling at a constant speed it is true that the forces on it are balanced (they are equal in value and opposite in direction), but they are NOT a NEWTON PAIR because they both act on the same object.

THIS is the hard bit and one which a few physicists may not answer correctly first time: what is the reaction to the weight of the parachutist?
well since his weight is the pull of the Earth on the parachutist, then the Newton reaction pair is the force with which he attracts the Earth. This may seem strange at first till you realise that a gravitational attraction must be an attraction between the TWO objects, So HE pulls the EARTH with the same force that the EARTH pulls HIM...a Newton Pair.

NOTE: (Thanks to Tom Mather): most people think that the reaction to the parachute is the UPTHRUST! (WRONG.....upthrust is TINY in air but a bit bigger and more noticeable in water!) This one was prompted by Tim Watson
and another anonymous reader pointed out that since a reaction is a force in the opposite direction by definition we should not really say "opposite reaction" as it is a sort of double negative, but I shall stick to the original wording!

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